Axial flow cooling for air-cooled engines

ABSTRACT

Cooling of existing air cooled engines uses fins and air flow essentially perpendicular to the axis of piston travel. By orienting the fins and air flow to be largely parallel to the axis of piston travel, far superior cooling can be achieved. The air flows thru grooves of relatively constant cross section for the entire flow path. All parts of the fins are reached about equally by the passing air. Both features contribute to improved heat conduction into the cooling air. Because air velocity over the fins is reasonably constant everywhere, less power is required to force the air thru the fin structure. Since the cooling power is ultimately taken from the engine output, this results in greater power being available to do productive work.

This application claims priority to co-pending U.S. provisional patent application No. 60/505,683, filed on 25 Sep. 2003, and incorporated herein by reference in its entirety.

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BACKGROUND

In general, air cooled engines have fins that require the cooling air to flow in a direction perpendicular to the axis of the cylinder. In a single cylinder engine, or an engine with a single bank of cylinders (such as a V2, a horizontally opposed 2, or a single bank radial), this is not a bad configuration. There is adequate space for fins and cooling air. Even a double bank radial works fairly well because the cylinders of the rear bank are oriented between the cylinders in the front bank, so there is adequate access for the cooling air to reach the aft cylinders. In the horizontally opposed air-cooled engines with 4 or more cylinders that are commonly used in automotive and aircraft applications, having the fins oriented perpendicular to the axis of the cylinders is a serious disadvantage. The air flow must be oriented parallel to the fins. That means either (1) the air flow thru the fins must be parallel to the axis of the crankshaft, which provides less cooling for cylinders behind the front cylinder, or (2) it must be perpendicular to both the crank axis and the cylinder axis, which is the orientation used in all modern applications.

It takes power to drive cooling air thru the fin structure. Ultimately this power must come from the engine being cooled. This reduces the useful power output of the engine, and the net efficiency of the engine. There has been a lot of effort for more than a century in designing efficient fin configurations for cooling engines with a minimum of lost power. To get efficient cooling, it is desirable to have the cross section of any given gap between fins to stay a constant area as the air passes thru the engine. With this configuration, the air moves with a constant velocity and a minimum of power is required to provide a given amount of cooling. In addition, the path length thru the engine should be minimized to maintain a thin boundary layer between the fin and the moving air.

Now consider the situation in standard down-draft (or up-draft) cooling where essentially all the air must pass between the cylinders. The air enters above the cylinder and head, which are typically 10 to 20 cm wide. Then it passes thru the gap between the cylinders, typically 1 to 2 cm wide. Then it is blown out beneath the cylinders and heads, again 10 to 20 cm wide. With careful duct design, the cooling air can be guided around the engine to pass over most of the fins, but the restriction at the passage between cylinders always increases the pressure required to force sufficient air thru the engine. The power required is the product of pressure times volume flow rate. The volume flow rate is fixed by the cooling requirements of the engine. If there is a restriction in the flow path that has half the area of the rest of the path, the flow velocity at that point will be twice as high as the velocity over the rest of the fin. Since pressure drop increases approximately with the square of flow velocity, the pressure drop per unit distance of air travel in the restriction will be four times as high as in the rest of the engine. With the air flow passing down between the cylinders, this is unavoidable. The result is excessive power required for cooling (very undesirable), and the possibility insufficient cooling under some or all operating conditions (even more undesirable).

It does little good to try to cool the engine from the “top” (further from the crank shaft). The rocker arms sit on top of the engine and that assembly introduces so much thermal impedance that it is impractical to cool the heads by using fins over the rocker arms. Porsche has developed a head in which the two valves are one above the other, as opposed to side by side, giving more space for fins and air passages between the heads. This requires a tricky valve linkage, and does nothing for the flow restriction between cylinders and the base of the heads.

For a specific example of present cooling problems, consider the Jabiru engine, built in Australia. The Jabiru has several desirable characteristics. It is a very compact engine for its power rating. Largely as a result of this, it is considerably lighter than other engines of similar power. Also, the small size makes the structure strong. Size and weight are important in many applications, and critical in aircraft. Strength is always desirable. A 6 cylinder Jabiru rated at 130 horsepower (100 kW) is essentially the same size as, and lighter than, a 4 cylinder Volkswagen producing half the power. There is no free lunch. The cost of the reduced size and weight of the Jabiru engine is that the compact design makes it essentially impossible to cool the engine when operated at rated power. Thru the remainder of this discussion, the Jabiru engine will serve as the model. However, all the results from this analysis of the Jabiru engine are obviously applicable to other in-line and horizontally-opposed air cooled engines.

FIG. 1 is a schematic representation of two adjacent cylinders of an engine, looking down the bore of the cylinder. Shown in FIG. 1 are the inner bore of the cylinder (1), the outer circumference of the cylinder (2) where it meets the head (which is essentially the same as the inside circumference of the head where it fits over the cylinder), the outer circumference of the head (3) where it surrounds the cylinder, and the outer perimeter of the fins (4) surrounding the head and cylinder. In addition, an engine needs head bolts (5). For strength, the head and cylinder both require a reasonable amount of material (6) surrounding head bolts (5). In the case of the Jabiru engine, there are six head bolts holding each head and cylinder together. This gives a great improvement in strength and rigidity over the usual situation where there are only four bolts for each cylinder.

Now consider the situation faced by the cooling air. The air typically enters at the top of the engine and flows down over the fins of the head and cylinder (downdraft cooling). The argument does not change much if the direction of flow is up from below the engine (updraft cooling). The air enters the fin structure in a region where the fins are typically 30 mm high and is squeezed between the cylinders where, in the case of the Jabiru engine, the fins are only 5 mm high. Thus, the air has to travel 6 times as fast while it is between the cylinders, which requires 36 times as much pressure drop per unit distance traveled, and 36 times the power per unit distance of flow. Ultimately, this power comes from the engine, and decreases the power available to do useful work. The problem is intensified in the Jabiru engine, where the use of six head bolts means that there is a long path length where the air must travel at high velocity. Also, when forcing air to flow around a cylindrical obstacle, the air flow tends to leave a dead air zone ahead of the center of the cylinder, and a much bigger dead air zone behind the center of the cylinder. Careful use of ducts to guide the air will reduce the sizes of these dead air regions, but it cannot eliminate them entirely. Another problem is that the conductivity of heat from the the metal fin to the air increases with increasing air velocity. Where the air moves slowly, a thick boundary layer forms, and conductivity into the air is low. In the situation shown in FIG. 1, the air travels slowly over 90% of the fin area, giving poor cooling, and where the air travels rapidly, there is not enough fin area to give adequate cooling.

Now consider the path length of the flow thru typical fins. In round numbers, this path length will be π times the average radius of the cylinder fins. If the cylinder has a bore of 100 mm, that is a radius of 50 mm. The cylinder wall has a thickness of about 5 mm, and the head must surround that by about an additional 5 mm. Thus, the radius to the base of the fins will be about 60 mm. If the fins are 30 mm high, the average radius of the fins becomes 75 mm. That gives a flow path length of 235 mm. This is a much longer path length than is desirable from purely thermodynamic considerations. Typical automotive radiators have path lengths of under 50 mm, and they usually have staggered fins within that distance. Aircraft oil coolers typically have air path lengths of 15 to 20 mm, with staggered fins within that distance. A flow path length of 235 mm is asking for thick boundary layers and lousy conductivity from the fin to the air.

BRIEF DESCRIPTION OF THE ILLUSTRATIONS

FIG. 1 is a view of an engine using conventional downdraft cooling, looking down the axis of piston travel.

All other figures depict engines, or parts thereof, using axial flow cooling.

FIG. 2 is a trimetric view a cylinder and head assembled to the side of a crankcase, showing a general view of the fins and air paths.

FIG. 3 is an end view of the head as seen from the cylinder, showing the distribution of fins around the combustion chamber.

FIG. 4 is a trimetric view of the head showing the fins on the intake port side and top of the head, and a few fins on the bottom of the head.

FIG. 5 is a trimetric view of the head showing the fins on the exhaust port side and top of the head, and some of the fins on the bottom of the head.

FIG. 6 is a top view of a section of two adjacent heads, showing the air flow path between them.

FIG. 7 is an end view of the cylinder as seen from the head, showing the fins surrounding the cylinder.

FIG. 8 is a trimetric view of the cylinder, showing how the fins are distributed along the length of the cylinder.

FIG. 9 is a detail of the ends of the cylinder fins, showing the results of machining them with sharpened edges.

SUMMARY

Most of the problems of both updraft cooling and downdraft cooling are eliminated by a novel cooling configuration, henceforth referred to as axial cooling, with the flow of cooling air traveling essentially parallel to the axis of the cylinder. Fins on the cylinder and head are oriented so they are essentially parallel to the axis of the cylinder. Cooling air is injected into the fin structure near the rocker arms. From there it flows over the head fins, then the cylinder fins toward the crank case. Ducts contain the air within the fin structures. The ducts may terminate some distance from the crank case, allowing the warmed air to escape. Better, the ducts may guide the warmed air toward the cooling air outlet, where it may be accelerated out of the engine compartment using exhaust augmentation. It is not necessary for the fins on the head to be aligned with the fins on the cylinder. In fact, it is undesirable for the two sets of fins to be aligned. Having a discontinuity in the fins between the head and cylinder disrupts the boundary layer, yielding improved heat transfer from the cylinder fins to the air. It is possible to pump the air in the opposite direction, from near the crankcase to the rocker arms. But, thermodynamically it is better to pass the coldest air over the region with the highest heat loading, the exhaust ports, near the rocker arms. Also, the ducts are easier to make and install if the air flow is from the rocker arms toward the crank case.

DETAILED DESCRIPTION OF THIS INVENTION

For purposes of illustration, the figures show axial cooling adapted to the Jabiru engine. Similar adaptations can be made to other air cooled engines. The compact design of the Jabiru engine provides a severe test for any cooling scheme. If it will work with the Jabiru engine, it will work with any engine.

FIG. 2 is a dimensionally correct view of a Jabiru head and cylinder bolted together, both with fins for axial flow cooling. Shown in FIG. 2 are the top and bottom edges of crank case (8) and an adjacent hole (9) for mounting a neighboring head and cylinder. Cylinder (10) is mounted to crank case (8) and is covered with fins (11) for axial flow cooling. Head (20) is mounted to cylinder (10) and is covered with fins (30) for axial flow cooling. The major features of head (20) are shown. These include the rocker arm housing (21), cutouts (22) in fins (30) for installing spark plugs (not shown), different cutouts (23) in fins (30) for installing head bolts (invisible in this view), and more cutouts (24) in fins (30) that allow clearance for the push rod tubes (not shown). Intake and exhaust ports are under the head, not visible. In operation, none of this is visible because the entire set of heads and cylinders is covered by a duct that constrains the cooling air to flow from beyond rocker arm housing (21), thru fins (30 and 11), toward crank case (8). Note that approximately half the air enters the fin assembly above the axis of cylinder (10), the other half enters below that axis. Thus, only half the total cooling air has to pass between adjacent cylinders. Also note that cylinder fins (11) are tall near head (20), where the heat load is great, and taper to zero toward crank case (8), where there is little heat load. Near crank case (8), the cylinder walls can be much thinner than they are near head (20) because there is little combustion pressure at the bottom of the piston stroke. Jabiru presently makes their cylinders this way. Between heads (20) there is a minimum space of about 11 mm where the heat load is greatest and there is 26 mm between the bottoms of cylinders (10), where there is little heat load. With downdraft cooling, it is necessary to cram all the useful air for head cooling thru an 11 by 70 mm space that is half full of fins. This gives less than 400 square mm of space that is high resistance because of the existence of the fins. With axial cooling, there is approaching 800 square mm of space between cylinders, carrying half as much air from above the engine to below it, with no fins to impede the flow. In addition, a real engine is not an infinite array of cylinders. The ducts can be shaped such that a significant fraction of the air cooling the top of the engine is guided around the ends of the engine, further reducing the flow required between cylinders.

FIG. 3 shows the surface of head (20) that faces cylinder (10 in FIG. 2). The entire periphery of head (20) is surrounded by fins (30). The bottoms of the channels (31) between fins (30) leave adequate material to provide strength around the edge of the cutout (25) where head (20) surrounds the top edge of cylinder (10 in FIG. 2) and around the holes (26) for the six head bolts (not shown). Although it is not obvious in this view, there is an important distinction between the intake valve side (27) and exhaust valve side (28) of head (20).

The entire periphery of head (20) is covered by fins (30), said fins being divided into several functional groups. Combustion chamber fins (32) cool the dome and upper edge of the combustion chamber. Exhaust side fins (33) cool the very high heat load of the exhaust port and the exhaust side edge of the combustion chamber. Intake side fins (34) primarily cool the intake side edge of the combustion chamber, the intake port not generating any heat. Bottom fins (35) cool the bottom edge of the combustion chamber. Notice that bottom fins (35) are cut out in two regions (24) to provide clearance for the push rod tubes (not shown). This leaves the fins in the corner (36) which provide some cooling to that edge of the combustion chamber but whose primary purpose is to provide a suitable quantity of cooling air to the region of cylinder (10 in FIG. 2) that is aligned with them. This is a convenient set of groups that facilitate discussion. All fins are part of one big block of metal and heat will tend to distribute itself to the coolest fins. The region of the exhaust port has the highest heat load in the entire engine. Obviously some combustion chamber fins (32) and bottom fins (35) that happen to pass close to the exhaust port will help cool the exhaust port.

In addition to the fins shown here, it is entirely possible, and desirable, to drill a set of holes vertically thru the metal separating the intake and exhaust ports. Such holes, properly aligned, can provide the air flow to the central couple grooves between combustion chamber fins (32). Blocking, or partially blocking, the entry to the grooves between these central fins, near rocker arm housing (21 in FIG. 2) will force air to flow thru these holes between the intake and exhaust ports. Thus, they will provide very significant cooling to the exhaust port and also provide thermal isolation between the exhaust port and the intake port (which should be kept cool for optimum engine performance). The present Jabiru heads use such a set of holes. This is not a new innovation herein, but it is applicable to the axial flow cooling design. Because such holes are used in the present Jabiru heads, it is not necessary to confuse these drawings by including said holes in these drawings.

Since the exhaust port quadrant of the head has about twice the thermal loading of any other quadrant, it might seem reasonable to make the head nonsymmetrical around the axis of the cylinder, with longer fins on the exhaust port side than on the intake port side. In fact, early Jabiru engines were made that way. Apparently Jabiru learned that this did not work well. Actually, that approach is counter productive. It results in little or no cooling on the edge of the combustion chamber at the intake port side. In addition, in an axial flow cooling system, as will soon be described, it results in lower air speeds over the fins on the the exhaust port side, with little or no increase in heat dissipation.

FIG. 4 is a trimetric view of head (20), seen from the intake port side. This is an identical view of head (20) as shown in FIG. 2, but the view is twice as large. Clearly it is unacceptable for the grooves between fins (30) to cut into any internal features of head (20). Air enters most combustion chamber fins (32) just above rocker arm housing (21). These cannot be cut any deeper without penetrating the oil supply manifold (29) for the rocker arms. A few combustion chamber fins (37) near the intake port can be cut much deeper, for considerable weight savings. Two of the grooves (38) between fins (37) cannot be cut as deep as might be expected without cutting into the intake port (internal to the head).

It is blatantly obvious that intake side fins (34) have no direct access to the cooling air input. This is a significant part of the design, not an unforeseen problem. The function becomes obvious in the discussion of FIGS. 5 and 6. Exhaust side fins (33) and corner fins (36) can be seen. They will be discussed further with FIG. 5.

Bottom fins (35) have a flow path length of only about 20 mm. They cool little more than the bottom edge of the combustion chamber. The volume between bottom fins (35) and rocker arm housing (21) is occupied by the intake pipe. Bottom fins (35) cannot be made longer on this side. There is no real need for the cooling on this side anyhow. On the exhaust side, where more cooling would be very desirable, the exhaust pipe does not allow a longer flow path thru bottom fins (35). In the middle, hidden behind rocker arm housing (21), resides the sixth head bolt. Access for machining that area and installing that bolt does not allow a longer flow path length for bottom fins (35) in that region. Although bottom fins do little cooling, they are necessary for delivering a proper quantity of air to cool the bottom of cylinder (10 in FIG. 2). Clearly, all head fins are working in parallel. The pressure drop across all flow paths is the same. Most of the pressure drop will occur across head (20). Cylinder fins (11) are relatively widely spaced, creating less pressure drop. In order to prevent the short path length of bottom fins (35) from carrying a disproportionally large fraction of the total air flow, the spacing between bottom fins (35) should be smaller than other fins, (creating more drag), bottom fins (35) should be thicker than other fins (occupying more of the cross section), and/or bottom fins (35) should be less high than other fins (giving less area for the air to flow thru).

FIG. 5 is the same scale and perspective as FIG. 4 except that head (10) is rotated 90° around a vertical axis. This is the best view to show how combustion chamber fins (32) wrap smoothly over the dome and upper edge of the combustion chamber. Note that combustion chamber fins (32) act as many little beams that strengthen and stiffen the dome of the combustion chamber. Fins cut for downdraft cooling tend to cut the dome of the combustion chamber away from the mounting surfaces, thus weakening it to the maximum possible extent. FIG. 5 also shows that all bottom fins (35) have the same short air flow path length. Note that near the side of head (20) it is possible to machine very short bottom fins that will dissipate some heat into the air that is flowing thru the region. These short fins lie outside the mounting of the exhaust pipe (not shown). Where cooling is critical, every little bit helps. Note that corner fins (36) also have a short air flow path length, about half as long as combustion chamber fins (32) and exhaust side fins (33), but about twice as long as bottom fins (35).

The dramatic thing shown in FIG. 5 is the height of exhaust side fins (33). Every exhaust side fin (33) in the central section is 25 mm high. Doing this requires that some grooves (39) be cut into the side of rocker arm housing (21), but not deeply enough to compromise its function. As the air approaches cylinder (10), it is pushed outboard, away from the axis of head (20), in order to clear the bottom of the combustion chamber. This is the region where side fins (33 and 34) can be only 5 to 10 mm deep. However, this is the region where there are intake side fins (34) on the adjacent head, which have no air supply other than that flowing out of mating exhaust side fins (33). Including the nominal clearance of nearly 2 mm between heads, the minimum total fin height for the air passages between heads is almost 12 mm, and the average is over 15 mm. The air path length thru the restriction is only a couple cm. Thus it does not represent a major impediment to the flow of air. Also, the restriction affects only the air flowing thru a few side fins, not the entire cooling air flow for head (20). The air flowing thru combustion chamber fins (32), bottom fins (35), and corner fins (36) is not affected by the close spacing of cylinders (10) and heads (20).

The air paths are more restricted where the air must flow around the cylinder bolts (hidden within the fins) at the bottoms of head bolt cutouts (23). As exhaust side fins (32) pass head bolt cutouts (23), the fin height is small to nonexistent. Closer to cylinder (10 in FIG. 2), exhaust side fins (32) are considerably taller, and the grooves between said exhaust side fins penetrate into head bolt cutouts (23). This allows air to flow thru head bolt cutouts (23), modestly cooling the walls of said cutouts, and providing additional air to fill the deeper grooves between the exhaust side fins (32). Despite the best of efforts, the regions of cylinders (10) immediately below the four side head bolts will receive the least air and will probably be the hottest areas on the cylinders, especailly near the head bolt beside the exhaust port. Still, with axial cooling, temperature variations in various parts of the engine are practically insignificant compared to the temperature variations encountered in the present Jabiru engines.

The air path thru the side fins is clearly shown in FIG. 6, which is a section view of exhaust fins (32) and intake fins (33) of an adjacent head, denoted as A-A in FIG. 5. This is a fairly average flow channel between the heads, neither the most restricted nor the least restricted. Air enters exhaust fins (32) from the right. As the air flows toward the left, the bottom (40) of exhaust side groove A-A curves to stay away from the edge of combustion chamber (42) and cylinder cutout (25). As the air is pushed up (in this view), it enters into the intake side groove (41) between intake side fins (33) of the adjacent head. For this particular groove, the minimum total fin height is 14.6 mm, and the total fin height is under 20 mm for a path length of about 22 mm. Thus peak flow velocity is less than double the minimum flow velocity, and it exceeds 125% of the minimum flow velocity for a distance of less than ⅓ of the total path thru head (20).

At one end of the engine, exhaust side fins (32) will not have mating intake side fins (33) to carry cooling air past the edge of combustion chamber (42). At the other end of the engine, there will be no exhaust side fins (32) to supply intake side fins (33) with cooling air. If the cooling air duct fits tightly to the heads at the ends of the engine, then exhaust side fins (32) suffer a severe restriction in their air flow path, and intake side fins (33) will receive no cooling air. It is a simple matter to shape the cooling air ducts to provide suitable passages and air flow to resolve this situation.

FIG. 7 shows the surface of cylinder (10) that faces head (20 in FIG. 2). FIG. 7 is symmetrical around the central vertical axis. To improve fin efficiency, cylinder fins (11) are tapered. For ease of manufacture, cylinder fins (11) are evenly spaced around cylinder (10). Neither of these characteristics are of fundamental importance to the principles of axial flow cooling. Because the heat loading of cylinder (10) is not as great as that of head (20 in FIG. 2), cylinder fins (11) are more widely spaced, saving weight and manufacturing cost and reducing the pressure drop across cylinder (10). The bottoms (12 and 13) of the grooves between cylinder fins (11) are aligned to the heights and slopes of the grooves between head fins (30 in FIG. 1). This minimizes turbulence and its resulting pressure drop. The bottoms of grooves (13) are high enough at the face of cylinder (10) that there is adequate strength around threaded holes (14) into which head bolts (not shown) are screwed. Inside circumference (15) of the bore of cylinder (10), outside circumference (16) of cylinder (10) near crankcase (8 in FIG. 2), and outside circumference (17) of cylinder (10) where it mates with head (20) are unchanged from the standard Jabiru cylinders. Cutouts (18) are provided for clearance around the push rod tubes (not shown). The envelope (19) of cylinder fins (11) is essentially the same as that of head fins (30), facilitating construction of the duct (not shown) surrounding cylinder (10) and head (20), and minimizing turbulence losses at the interface between the two.

Note that both the head (as shown in FIG. 2) and the cylinder (as shown in FIG. 7) have cutouts in the fins where the push rod tubes reside. Note also that the cooling air flow is coaxial with the push rod tubes. This will make a significant contribution toward meeting the total oil cooling requirement. In most air cooled engines, the engine cooling air flow largely misses the push rod tubes and makes no significant contribution toward cooling the oil.

FIG. 8 is a trimetric view of cylinder (10), identical to that in FIG. 2, except twice as large and not partially obscured by head (20 in FIG. 2). FIG. 8 shows fins (11), grooves (12 and 13), threaded holes (14), diameters (15, 16, and 17), and cutouts (18), as in FIG. 7. To reduce drag on the cooling air, cylinder fins (11) are full height only near head (20), where the heat loading is greatest. They taper to zero height near crankcase (8 in FIG. 2). For ease of manufacture, the envelope of this tapered region is a cone. The bottoms of grooves (12 and 13) lie on circumference 16 for most of the length of said grooves. As grooves (12 and 13) approach head (20), the bottoms of said grooves curve to make a smooth transition to the grooves between head fins (30 in FIG. 2). None of these features is essential to axial flow cooling, but they do improve performance and/or reduce manufacturing cost.

Comparing FIGS. 3 and 7, it is obvious that there is no coherence whatsoever between the locations and orientations of head fins (30) and cylinder fins (11). This is desirable in that it interrupts the boundary layer that forms over a continuous surface, said boundary layers reducing the thermal conductivity from the surface to the air flowing over it. With the discontinuity in the fin surfaces between head (20) and cylinder (10), when the cooling air reaches cylinder fins (11), the boundary layer has to start over at essentially zero thickness. Thus the flow path length over both head fins (30) and cylinder fins (11) is in the range of 70 mm. While this is longer than ideal, it is a great improvement over the 235 mm path length typical of the present Jabiru head design. A disadvantage of this discontinuity is that air striking the blunt faces of cylinder fins (11) stalls, increasing drag and decreasing effective cross sectional area of the grooves between cylinder fins (11) and head fins (30) near the transition between head (20) and cylinder (10).

To greatly reduce this effect, the leading edges of cylinder fins (11) are sharpened, as shown in FIG. 9. This is a detail showing only a few fins. An easy way to create the sharp leading edge is with two small saw cuts for each fin using a saw blade that cuts a round bottomed groove. FIG. 9 shows one threaded head bolt hole (14), cylinder bore (15), and the outside circumference (17) of the cylinder insert into head (20). Between fins (11) are grooves (12 and 13), showing both depths, demonstrating that the leading edges of all fins (11) can be sharpened. In FIG. 9, the leading edge (45) is brought to a knife edge. This is probably not a satisfactory solution, for safety reasons, but it demonstrates the process and it is by far the easiest to draw and understand. 46 is the intersection between the original surface of fin (11) and the slope to leading edge (45). 47 is the intersection between the flat side of the cut and the round bottom. 48 is the intersection between the round bottom of the sharpening cut and the round bottom of the original groove (12 or 13). 49 is the intersection between the face of cylinder (10) and the round bottom of the sharpening cut.

It is also desirable to sharpen the trailing edges of the head fins. Similarly, it is desirable to offset and sharpen the edges of exhaust side fins (32) and intake side fins (33) to break up the boundary layer and improve flow between them. However, both these steps are significantly more difficult to implement, and the improvement is significantly less than is gained with sharpened cylinder fins. Consequently it is probably not worthwhile except in extreme conditions such as airplane racing.

Ducts for containing cooling air within the axial flow fins are considerably simpler than ducts presently used in low drag down-draft cooling installations. A three piece duct is adequate for each side of the engine. One piece extends over the cylinders and heads from the crank case to the spark plugs. A second piece extends under the cylinders from the intake and exhaust pipes to a few cm from the crank case, leaving ample space for the hot air to exit downward. Ideally, this second piece is connected to a plenum under the engine which guides the heated air toward the outlet port. The third piece of duct extends from the spark plugs, over the rocker covers, to the intake and exhaust pipes, connecting to the first and second pieces. The third piece also incorporates the nozzle that picks up the intake cooling air. A wide variety of duct configurations will work with an axially cooled engine. This is just one example of a simple, effective duct that allows easy access to the engine.

Cooling ducts for axial flow cooling are not shown. The required shape of such cooling ducts is obvious. An optimized duct will contain a diffuser and turning vanes. These features are well known in duct design.

Definition

Fin Efficiency: The heat flow rate from a given fin divided by the heat flow rate that would occur if the fin had an infinite thermal conductivity, all other parameters remaining unchanged.

High fin efficiency sounds good, but it requires a lot of fin material and large volumes of space in which to locate it. In real situations, for almost any definition of “optimum”, the most desirable fin efficiencies are in the vicinity of 70%.

Experimental Results

Thermal measurements on a Jabiru engine in a laboratory indicate that it is probably impossible to operate the engine continuously at rated power in any existing airplane. These measurements, with very good ducts distributing cooling air over the heads, indicate a quiescent temperature rise of 300 C when operated in a normal small plane. There is not enough pressure available at speeds small airplanes can realistically achieve to force enough air thru the engine to keep it cool, especially on a warm day. It will barely survive at 70% cruise power. The condition of used heads from Jabiru engines is testimony to the difficulty of keeping the engine at a reasonable operating temperature. There are two major problems, fin area and air flow velocity. A Jabiru head has about 1500 cm² of surface area actually exposed to moving air, and over much of that area the air movement is sluggish. The fin efficiency of the Jabiru engine is very high, above 95%, but this does little good because there is so little fin area and the cooling air cannot effectively reach much of the area that does exist.

By comparison, the head demonstrated herein has a fin surface area of about 2700 cm², 1.8 times as much, all fit within the same volume. More important, the air flows evenly over essentially all of that area cutting thermal resistance from the fin to the air to a fraction of the value in the present Jabiru engine. This is important, but even more important is the fact that the pressure required to drive air thru the head fin assembly is dramatically lower. Tests on a flow bench with a mockup of the head fins demonstrated herein show that for any given volume flow rate, the pressure drop of this design is well under 10% that of the present Jabiru design. Thus there is adequate air pressure available to cool the head to a comfortable temperature even at full power and low speeds, as in a prolonged, steep climb. Not only does this promote engine reliability, it dramatically reduces cooling drag, which takes a significant fraction of the total engine output power. The result is higher speed and better fuel economy.

The fin efficiency in this design is over 75%. This is a little higher than optimum, but cutting thinner fins and making more of them would be difficult.

Range of Applications

This patent is a detailed description of axial cooling applied to the Jabiru engine. The Jabiru was selected as a model because its compact design exacerbates the cooling problems present in all air cooled engines. Clearly a similar design process will result in improved cooling for other existing engines, and new engines that may be designed in the future.

Axial cooling is most applicable to horizontally opposed engines of more than two cylinders. It can be used in 2-cylinder horizontally opposed engines, but the advantages are limited. At this time, there are very few in-line air cooled engines. Axial cooling is certainly appropriate for in-line engines, if anyone is interested. Most V engines of more than two cylinders are water cooled, but an axial-flow, air-cooled V engine is certainly possible, with air entering above the engine, flowing down thru the head and cylinder fins, and exiting at the two sides of the crank case. In radial engines, as in 2-cylinder horizontally opposed engines, axial flow cooling could be used, but the advantages are limited. The real advantages of axial flow cooling occur in configurations where there is limited space for air to pass between adjacent heads and cylinders, and it is advantageous in any such engine.

Axial flow cooling is applicable to stationary installations with a fan providing the driving power to the cooling air, to mobile installations where the motion of the vehicle causes the cooling air to flow over the engine, and to mobile installations where a fan (or propeller) is used to augment the airflow caused by the motion of the vehicle. As mentioned above, Exhaust augmentation of the cooling flow is also possible and desirable with axial flow cooling. 

1 An air cooled engine with at least two cylinders, in which more than half of the head cooling fins are oriented within 30° of being parallel to the direction of piston travel. 2 An air cooled engine as in claim 1, in which more than half of the head cooling fins are oriented within 20° of being parallel to the direction of piston travel. 3 An air cooled engine as in claim 1, in which more than half of the head cooling fins are oriented within 10° of being parallel to the direction of piston travel. 4 An air cooled engine as in claim 1, in which more than half of the head cooling fins are oriented within 5° of being parallel to the direction of piston travel. 5 An air cooled engine as in claim 1, in which more than half of the head cooling fins are oriented within 2° of being parallel to the direction of piston travel. 6 An air cooled engine as in claim 1, in which more than half of the head cooling fins are oriented within 1° of being parallel to the direction of piston travel. 7 An air cooled engine with at least two cylinders, in which more than half of the cylinder cooling fins are oriented within 30° of being parallel to the direction of piston travel. 8 An air cooled engine as in claim 7, in which more than half of the cylinder cooling fins are oriented within 20° of being parallel to the direction of piston travel. 9 An air cooled engine as in claim 7, in which more than half of the cylinder cooling fins are oriented within 10° of being parallel to the direction of piston travel. 10 An air cooled engine as in claim 7, in which more than half of the cylinder cooling fins are oriented within 5° of being parallel to the direction of piston travel. 11 An air cooled engine as in claim 7, in which more than half of the cylinder cooling fins are oriented within 2° of being parallel to the direction of piston travel. 12 An air cooled engine as in claim 7, in which more than half of the cylinder cooling fins are oriented within 1° of being parallel to the direction of piston travel. 